Apparatus for the liquid toning of latent electrostatic images



Nov. 3, 1964 RKS 3,155,546.

J. F. DI APPARATUS FOR THE LIQUID TONING OF LATENT ELECTROSTATIC IMAGES Filed Sept. 24, 1962 2 Sheets-Sheet 1 INVENTOR.

. JOHAN FT DIR/(S BY Q$W-- ATTORNEY Nov. 3, 1964 J. F. DIRKS 3,155,546

APPARATUS FOR THE LIQUID TONING 0F LATENT ELECTROSTATIC IMAGES Filed Sepiu 24, 1962 2 Sheets-Sheet 2 FIG. 3

[N VEN TOR.

JOHAN F. DIR/(S W Qajakmgh A TTORNEV United States Patent The Plastic Co ng Corporation Slept. 24, @523, Ser. No. 225,476

Claims. (Cl. 11S-637) This invention relates to an improved apparatus for the application of a liquid toner to a latent electrostatic image on a photoeleotrostatic sheet, to convert that image into a permanent, visible image.

In photoelectrostatic reproduction, an optical image is reproduced directly on a film or coating of a photoconductive pigment such as zinc oxide, dispersed in an insulating matrix such as a synthetic resin, carried by a sheet of paper or other electrically-conductive baclo ing. In this process, the photoelectrostatic coating is given a negative electrostatic charge while protected from light, and then exposed to an optical light image of the subject matter to be reproduce The negative electrostatic charge on the coating is discharged in the areas struck by light, while being retained in the areas protected from light, thereby producing an electrostatic reproduction of the optical image. This electrostatic image is then converted to a visible image by either the application of a dry toner and then fusing the ton r, or of a liquid toner followed by the evaporation of the volatile solvents in which the toner is carried.

Liquid toners for the development of electrostatic images comprise a dispersion of a pigment in a volatile liquid having a high-dielectric strength and a highvolume resistivity. The dispersed particles may carry either a positive or a negative electrical charge, depending upon their chemical compositions. The high-dielectric strength of the volatile liquid of the liquid toner preserves the electrostatic image, and permits the deposition of the dispersed particles to form a permanent, visible Liquid toners are described and exemplified by US. Patent Number 2,907,674, issued October 6, 1959, to Metcalfe and Wright.

The development of the electrostatic image by the toning may be either positive or negative. The terms positive toning and negative toning have acquired a particular meaning, and it is in the following sense that these terms are hereinafter employed:

(1) The positive toning of an electrostatic image is the deposition of electrostatically-attractable, opaque particles on the image in an amount somewhat proportional to the charge retained by the image.

(2) The negative toning of an electrostatic image is the deposition of electrostatically-attractable, opaque particles on background areas of an image in an amount somewhat proportional to the charge retained by the image.

A liquid toner having positively-charged, opaque particles is used in the positive toning of a negative electrostatic image. 0n the other hand, a liquid toner having negatively-charged, opaque particles is used in the negative toning of a negative electrostatic image.

It has been recognized that the application of an electrical potential lower than that of the electrostatic image during the toning, can improve the resulting visual image. Such an electrical potential has been termed a bias voltage. The bias voltage applied in the toning operation may be the same as that of the electrostatic image, or of reverse polarity. The application of a negative bias voltage during the toning of a negative electrostatic image, which may be termed a backward bias, tends to retard the deposition of positively-charged particles on the image and to produce cleaner background areas in the resulting visible image. Conversely, the application of a positive bias voltage during the deposition of positively-charged toner particles to a negative electrostatic image, which may be termed a forward bias, increases the over-all density of the visible image and is valuable in bringing out fine and obscure detail.

Bias voltage has heretofore been applied to a sheet carrying a latent electrostatic image, during the toning of the image with a liquid toner, by applying voltage of one polarity to the surface of a sheet carrying an image, while grounding both the reverse surface of the sheet and the opposite pole of the source of DC. voltage. Bias voltage has been applied in this manner through rolls between which the sheet passes before and after the application of liquid toner to its surface bearing the electrostatic image.

This method for the application of bias voltage has two serious disadvantages, particularly when used in compact apparatus such as one adapted for the reproduction of ofiice copies on cut sheets of photoelectrostatic paper. In such apparatus, the trailing portion of the surface of a photoelectrostatic sheet is being charged while the leading edge of the sheet is being toned. The electrostatic charge being imposed on the trailing end of the sheet, prior to its exposure to an optical image, affects the potential of that part of the sheet which is between the bias rolls, upsetting the balance of the bias potential. The result is an objectionable darkening of the background of the leading edge of the sheet, causing the finished copy to be unsightly. The second disadvantage which has appeared in such toning arises from the fact that the bias voltage is continuously applied to the bias rolls. In normal usage, the time during which a cut sheet is passing between the bias roils is a minor fraction of the total time during which the apparatus is turned on with the bias voltage being applied to the bias rolls. During the periods when a sheet is not passing between the bias rolls, the rolls are wet with the liquid toner as a result of the toning of a preceding sheet, and the toner particles in the liquid toner tend to migrate to the roll which contacts the image side of the following sheet. This accumulation of the toner particles on this bias roll tends to deposit on the image surfaces of succeeding sheets, causing them to have a dirty, unsightly appearance. Further, such continuous application of bias voltage tends to upset the electro static balance within the toner liquid, itself, which degrades its performance.

It is an object of this invention to provide an apparatus for the application of bias voltage during the liquid toning of an electrostatic image, which avoids the forementioned disadvantages and provides a uniform background and sharp contrast in the visible image.

A further object is to provide an apparatus for the application of bias voltage which permits the use of a lower bias potential and gives a better clean-up of the image-bearing surface of the toned copy, while avoiding the adverse effect of the application of an electrostatic charge to the trailing end of the copy sheet during the toning of the forwardly end of the sheet.

Another object is to provide an apparatus for the application in which a bias potential is applied to the bias rolls only during periods when a cut sheet is being toned, thus limiting the period during which the bias is applied to no more than about twice the period during which a sheet is actually passing between the rolls.

A still further object is to provide apparatus for the application of bias potential utilizing a pair of rolls, one of which is a resilient, conductive roll which functions both to apply a bias potential to a copy sheet wet with liquid toner and to squeeze the excess liquid toner from 6 the sheet, eliminating the necessity for an additional pair of rolls to remove the excess toner liquid and avoiding the use of a semiconductorroll, which is difficult to manufacture.

Other objects and advantages of this invention will become apparent from the detailed description which follows.

In the apparatus in accordance with this invention, a flexible base sheet carrying a negative electrostatic image is toned with a liquid toner by passing it between a first pair of rolls, the surface of the sheet bearing the electrostatic image treated with a liquid toner to uniformly wet the electrostatic image, and then passed between a second pair of rolls. The second pair of rolls are, respectively, electrically connected to opposite poles of a source of DC. voltage, and at least one of them is connected through a resistor to the source of DC. voltage. Each of the rolls may have a resistor in the line by which it is connected to the source of DC. voltage, These rolls are, otherwise, electrically insulated from the remainder of the apparatus.

The roll, of this second pair of rolls, which comes into contact with the face of the sheet carrying the toned electrostatic image is, preferably, a smooth, rnetal roll, since such a roll has little or no tendency to pick up portions of the toner from the image and reprint it elsewhere on the sheet. The second roll of this second pair of rolls, which contacts the back of the sheet, preferably has an electrically-conductive, flexible surface to permit the rolls to be brought into tight contact which will remove excess toner from the sheet. The flexible surface of this roll may, for example, be an electricallyconductive rubber. In the embodiment of this invention in which only one of the two rolls of this second pair of rolls is connected through a resistor to the source of D.C. voltage, I prefer to connect the roll having the flexible surface in this manner.

The first pair of rolls between which the sheet is passed before being wet with theliquid toner, may both be electrically non-conductive rolls. However, in a preferred embodiment of this apparatus, I prefer to utilize an electrically-conductive roll to contact the back of the sheet, which is electrically connected to the positive pole of the source of DC. voltage but, otherwise, electrically insulated from the remainder of the apparatus. In this preferred embodiment of thisapparatus, the application of the positive electrical potential to the back of the sheet before it comes into contact with the liquid toner, neutralizes or at least materially reduces any negative electrostatic charge carried by the back of the sheet, and minimizes any pick-up of toner pigment when it becomes'moist with the toner liquid during the toning of the electrostatic image on the faceof the sheet.

In the use of this apparatus, it is convenient to pass the sheet between the first and second pair 'of rolls in a manner such that the face of the sheet which carries the electrostatic image faces generally upwardly as it passes between the two pair of rolls, to facilitate the uniform application of the liquid toner to the image-bearing surface. Regardless of the position of the face of the sheet bearing the electrostatic image when wet with liquid toner, the first and second pair of rolls are arranged as described hereinbefore with respect to the face and back of the sheet.

In the use of this apparatus, application of the bias potential to the second pair of rolls, preferably, is limited as nearly as practical to the period of the toning of an electrostatic image on a photoelectrostatic sheet, and the time during which the bias voltage is applied to that pair of rolls when a sheet is not passing through the nip between them is kept to a minimum. This avoids a flow of electrical current between the rolls, which causes an excessive transfer of the toner pigment from the roll which normally contacts the back of the sheet to that which contacts the face of the sheet, as well as an excessive flow of current through the bias system.

This apparatus may be used in the liquid toning of electrostatic images on successive photoelectrostatic sheets, or on a continuous web of a photoelectrostatic sheet material from a roll. It is particularly well adapted for the liquid toning of images on cut sheets in an office-copy machine. In the use of this apparatus, the electrical connections which apply the bias voltage to the rolls of the apparatus are closed by a first switch which is tripped by the forward edge of a sheet when it is in the vicinity of the first pair of rolls and, preferably, as it emerges from that pair of rolls, maintained in closed position by the passing surface of the sheet, and then opened when released by the passage of the trailing edge of the sheet. A second switch, in parallel with the first switch, is located in a position to be closed by the forward edge of a sheet coming into the vicinity of the second pair of rolls and, preferably, as it emerges from that pair of rolls, to be kept closed by the passing of the sheet, and then to open when the trailing end of the sheet has passed its tripping arm.

Either positive or negative toning may be carried out by the use of this apparatus. In positive toning, a liquid toner carrying positively charged, opaque particles is applied to the sheet, while in negative toning, a toner liquid carrying negatively-charged particles is applied thereto. In either the positive or the negative toning, a backward bias voltage or a forward bias voltage may be utilized. a

In the application of a liquid toner carrying positively charged, opaque particles in positive toning, a backward bias voltage preferably is applied by connecting the roll of the second pair of rolls which comes into contact with the surface of the sheet carrying an image, to the negative pole of a source of DC. voltage. The other roll of this pair of rolls is connected through a resistor to the positive pole of the source of DC. voltage, while the conductive roll of the first pair of rolls is directly connected to this positive pole. For the application of a forward bias in the positive toning of a negative image, these connections are reversed.

The electrical connections for the application of a backward bias in the negative toning of a negative image by the application of a liquid toner carrying negative particles, are identical with those for the application of a forward bias in the positive toningof such an image. Conversely, the electrical connections for the application of a forward bias in the positive toning of a negative image, are identical with those for the application of a backward bias in the positive toning of such an image.

The bias voltage, whether positive or negative, which is applied in the liquid toning of an electrostatic image by the apparatus in accordance with this invention, will normally fall within the range of about 10 volts to about 300 volts. The resistor, through which the roll of the second pair of rolls comes into contact with the back of the sheet, is within the range of about 1,000 ohms to about 1 megohrn.

As will be appreciated from the foregoing description of the method carried out by the apparatus in accordance with this invention, no ground connection is included in the electrical circuits utilized. The bias voltage employed may be termed a floating bias voltage, as distinguished from the methods heretofore employed in which a ground connection is included in the electrical circuit utilized.

As will be apparent from the foregoing description, the apparatus in accordance with this invention for the toning of an electrostatic image by the use of a liquid toner consists essentially of two pair of rolls; a source for liquid toner; a source of a bias potential; an electrical resistor; electrical connections between the source of bias volt-age, the resistor and the said rolls, as described hereinafter; and a means for transporting photoelectrostatic sheets between the first of the two pair of rolls adjacent the source of liquid toner, and between the second pair of rolls.

Both of the first pair of rolls may be insulating rolls. In this apparatus, the path of travel of a sheet passing from the nip between the first pair of rolls to the nip between the second pair of rolls is less than the length of the sheet to be toned. Alternatively, the roll of the first pair of rolls which comes into contact with the surface of a sheet carrying an image is non-conductive and may, for exann ple, be a rubber roll. The second roll of this pair of rolls which comes into contact with the back of the sheet is a conductive roll and is electrically connected directly to the source of bias voltage and may, for example, be made of a structural metal. In apparatus in which a negative electrostatic image is formed, this second roll of the fi st pm'r of rolls is connected to the positive terminal of the source of DC. potential.

Both rolls or the second pair of rolls are conductive rolls. As noted in the description of the m ed, the one of these rolls which come into contact wit do back of the photoeiectrostatic sheet desirably has a no. ble surf ce and may, for example, be made of a conductive rubber having a resistance of about 10 ohms, and not more about l0 ohms. The electrical resistor is in the electrical circuit between the source or" DC. peter l and one of the rolls of the second pair of rolls. A second electrical resistor may be interposed in electrical connections between the second roll of the second pair of rolls he source of 11C. voltage. When using only one elect ical resistor in the electrical circuit, I prefer to place it in the electrical circuit between the roll the second pair of rolls which contacts the back of the copy sheet and the source of DE. voltage.

The poles of the source of DC. voltage which are electrically connected to the respective rolls of the second pair of rolls, are determined by the type of bias which is to be applied to the toned image on a copy sh i.c., whether it is desired to apply a backward or a forward bias potential. In the application of a backward bias, which is the most frequently used, the roll of the second pair of rolls which contacts the image-bearing surface of the copy sheet is connected to the negative pole of the sou 3c of DC. voltage, the other roll of this second p tr of rolls, i.e., the one which comes into contact with the heel: of the copy sheet, is connected to the positive pole of the source of DC. voltage. As noted above, at least one at these electrical connections is through the resistor. For the application of a forward bias to the toned copy sheet, these electrical connections are reversed.

A high-desirable feature of this apparatus is a pair of electrical switches which are connected in parallel in the electrical connection between the source of voltage and one of the rolls of the second pair of rolls, and are adapted to be closed by the leadirn edge of a copy sheet, to be kept closed by the passing of the sheet, and then to open as the trailing edge of the sheet passes and to remain open when no sheet is passing through the toning apparatus.

One electrical switch of this pair or" swit hes has its tripping arm or other activating men. s positioned in the vicinity of the first pair of rolls in a location with respect to the path of travel of a copy sheet, wl ch causes inc leading edge of the st"- t to close the swi and to it closed until the trailing edge of the sheet passes it. A convenient location for the activating means is adjac t the exit side of nip between the first pair of rolls. The second of the switches is located in the vicinity of the sec- 0nd pair of rolls, with its activating means in position to cause the leading edge of a copy sheet to close the switch the passing surface of the sheet to keep the switch closed until its trailing edge passes the activating mes The activating means of this switch is preferably loca adjacent the exit side or" the nip or the second pair of to The distance along the path of travel of a copy sheet between the activating means of the first switch and that of the second switch must be less than the length of the copy sheet to be toned, in the direction of its travel apparatus.

The closing of the first of these electrical switches as the leading edge of the sheet enters this toning apparatus, applies the bias potential to this toning apparatus. This bias potential is maintained until the trailing edge of the sheet passes out of the apparatus, since the second switch is closed by the leading edge of the sheet before the first switch opens.

While I prefer to use this improved feature of my apparatus for the interruption of the application of the bias potential in conjunction with my improved method and means for the application of the bias potential to the copy sheet, it will be understood that it may also be used to advantage in conjunction with the prior art means for the application of a bias potential to the copy sheet. Similarly, it will be understood that this improved feature of my method and apparatus is not essential to the improved method and means for the application of the bias potential to the sheet being toned. In fact, it is unnecessary in apparatus which is designed to tone long webs of copy paper on a more-or-less continuous basis.

A preferred embodiment of the apparatus in accordance with this invention, which is adapted for the toning of cut sheets of copy paper as an integral part of an oflicecopy apparatus, will now be described with reference to the accompanying drawings, in which like reference charactors are used to refer to like parts wherever they may occur. in the drawings:

FIGURE 1 is a perspective view of the toning apparatus in accordance with this invention;

FIGURE 2 is a longitudinal, cross-sectional view of the toning apparatus, taken along the section line 2-2 of FTGURE 1, and

FiGURE 3 is an electrical wiring diagram of a preferred embodiment of the electrical circuits utilized in this toning unit.

The apparatus illustrated by FZGURES l and 2 is suitaole tor the liquid toning of an electrostatic image using on er a positive or a negative liquid toner with the application of either a forward or a backward bias voltage during the toning of the sheet. The electrical circuit illustrated by FlGl-IUE 3 is adapted for the application of a forward bias when applying a negative toner to a nega tive electrostatic image or a backward bias when applying a positive toner to a negative electrostatic image.

Inasmuch as positive images are usually desired in copies, positive toners are more frequently used than negative toners for the toning of negative electrostatic images and clean backgrounds are usually more important than intensified images, the following description of the figures of the drawings will be directed to the use of a positive toner while applying a backward bias in the toning of a negative electrostatic image.

Referring specifically to PTGURES 1 and 2, the numeral in designates a continuous belt, carried by the rolls 11 and 12, at least one of which is driven by a suitable means, not shown by the drawings, to cause the movement of the belt in the direction indicated by the arrow. The roll 13 is adjustable to bear against the inner side of the belt 1b to keep it taut. The rolls 1d and 15 are located below the belt with the nip between them positioned to receive the edge of a sheet carried by the downwardly-movin span of the belt lit. The rolls 14 and 15 are rotated at the same speeds in opposite directions, as indicated by the arrows, by suitable drives not hown by the drawings, to move a sheet entering the nip between them in a downwardly direction.

The curved sweep 16 has its concave side facing upwardly, with one end located below the roll 15. It is in a position to receive the forward end of a sheet and guide the sheet along an arcuate path. The rolls J7 and 18 are located adjacent the opposite end of the sweep 16 and are rotated, at the same speeds, in opposite directions indiated by the arrows, by means not shown by the through the drawings. The nip between the rolls 17 and 18 is located with respect to the edge of the sweep 16, and the direction of their rotation is such that a sheet propelled along the sweep 16 by the rolls 14 and 15 enters the nip and is pulled forward by the rolls 17 and 18. The speed of rotation of the rolls 17 and 18 is the same as that of the rolls 14 and 15, with the result that a sheet of paper carried by these pairs of rolls follows the curature of the sweep 16. g V 7 Referring specifically to FIGURE 2, it will be seen that the curved sweep 19, having its concave side facing downwardly, has one end positioned above the rolls 17 and 18. The sweep 19 extends in the forwardly direction of the travel of a sheet passing through the system to a position above a series of conveyor belts ZIP-2t) carried at one end by the roll 21 and, at the other, by a roll not illustrated by the drawings. The sweep 19 is shown by FIGURE 2, but is omitted from FIGURE 1 to avoid obscuring other details of the apparatus.

The conduit 22 is provided with a series of perforations 24A-24A on its lower side, and is located above the sweep 16. This conduit is connected with a source of liquid toner which provides the liquid toner to the conduit at any time that a sheet is passing through the mechanism. The toner sprays through the perforations 22A--24A in the conduit 22, onto the surface of a sheet passing over the sweep The excess toner flows 011 of the ends of the sweep 16, onto the tray 23, and into the tank 24 from which it is recirculated to the conduit 22.

The electrical switch 30 is carried by the structural member 31 of the apparatus with its spring-loaded trip arm 32 in a position to be moved into the depressed position shown in broken outline by FIGURE 2, to close the switch by the edge of a photoelectrostatic sheet passing out of the nip betweenthe rolls 14 and 15, and maintained in that position by the passing surface of the sheet until its trailing edge passes the arm. The spring loading of the trip arm has its tension directed to open the switch after it is released by the passing of the trailing edge of the sheet. The switch 30 is preferably a sensitive microswitch which can be closed and kept closed by a light pressure on its trip arm 32.

The switch 33 is carried by the structural member 34 of the apparatus, with its spring-loaded trip arm 33 in a position to be moved into the depressed position shown in broken outline by FIGURE 2, to close the switch by the edge of a sheet passing from the nip between the rolls 17 and 18, and to be maintained in that position by the passing surface of the sheet. The spring loading of the trip arm 35 has its tension directed to open the switch as it is cleared by the trailing edge of the sheet. The distance between the trip arm 32 and the trip arm 35 around the convex side of the sweep 16 is less than the length of a sheet of copy paper with which the apparatus is to be used. As in the case of the switch 30, the switch 33 is preferably a sensitive micro-switch which can be closed and kept closed by a light pressure on its trip arm 35; p

The roll 14 is an insulating roll, while the roll 15 is an electrically-conducting roll which is electrically connected through the bar 36 and the electrical line 37, shown by FIGURE 1, to the positive pole of a source of D.C. voltage 38, as shown by FIGURE 3 and is, otherwise, electrically insulated from the remainder of the apparatus. The source 38 of D.C. bias voltage is one capable of supplying voltage within the range of about volts to about 300 volts. It is, desirably, one which permits the convenient adjustment of the voltage which it supplies within this range.

The roll 17 is an electrically-conductive roll made, for example, of a structural metal which, preferably, has an electrically-conductive, resilient, cylindrical surface such as, for example, one of an electrically-conductive rubber or other electrically-conductive, resilient material which is not attacked by the volatile liquid of the liquid toner. This roll may, for example, have a resistance Within the range of about 10 ohms to about 10 ohms. As shown by FIGURES 1 and 3, this roll is electrically connected through the bar 38 and the electric line 39 to the resistor 40. The resistor 40 is, in turn, connected by the line 37 to the positive terminal of the source of D.C. voltage 38.

The roll 17 is, otherwise, electrically insulated from the.

remainder of the apparatus.

The roll 18 is an electrically-conductive roll made, for example, of a structural metal. As shown by FIGURES 1 and 3, the roll 18 is electrically connected by the bar 41 and the electric line 42 which,'as shown by FIGURE 3, is, in turn, connected to one of the terminals of the switches 30 and 33, respectively. The other terminal of the switches 30 and 33 are, respectively, connected by the line 43 to the negative pole of the source 38 of D.C. voltage. It will be observed from FIGURE 3 that the switches 30 and 33 are in parallel, and that the closing of either or both of these switches closes the electrical circuits between the source 33 of D.C. potential andeach of the rolls 15, 17 and 18.

In the utilization of the apparatus illustrated by FIG- URES 1, 2 and 3 in the liquid toning of photoelectrostatic sheets bearing electrostatic images, the sheets are successively transported to the nip between the rolls 14 and 15 by the belt 10, after the electrostatic image has been produced on the surface of the sheet by the successivesteps of exposing its photoelectrostatic surface to a corona discharge while it is protected from light to impose an' electrostatic charge thereon, and then exposing the charged surface to an optical image. The face of the photoelectrostatic sheet carrying the electrostatic image comes into contact with the roll 14'. Stated in anotherway, the back of the sheet is in contact with the belt 10. The apparatus, including the corona discharge unit, the optical system and the transport means involved in these steps are known to the art and are not a part of this invention.

As the sheet emerges from the nip between rolls 14 and 15, its leading edge comes into contact With the tripping arm 32 of the switch 30, closing the switch 32 connecting the rolls 15, 17 and 18 to the source of D.C. voltage 38. The trip arm 32 i maintained in its tripped position by the passing surface of the sheet, keeping the electrical circuits of the bias voltage closed. The sheet is guided along an arcuate path by the sweep 16, and passes under the toner conduit 32 from which liquid toner is being sprayed. The liquid toner wets the surface of the photoelectrostatic sheet which carries the electrostatic image which is facing upward as it passes under the spray. The excess liquid toner flows off of the edges of the sheet, and is recirculated to the conduit 32 from which it emerges as a spray.

The positive charge on the roll 15 which is furnished to that roll by the closing of the switch 36, neutralizes any negative electrostatic charge carried by the back of the sheet. This neutralization of the charge on the back of the sheet prevents or materially reduces the discoloration of the back of the sheet by a pick-up of toner pig ment when it becomes moist with the toner liquid during the toning of the electrostatic image on the face of the sheet.

The photoelectrostatic sheet is propelled along the sweep 16 and moves its leading edge into the nip between the rolls 17 and 18, which continue the movement of the sheet. As the leading edge of the sheet emerges from the nip between the rolls 17 and 18, it comes into contact with the tripping arm 35 of the switch 33 which, like the switch 30, closes the electrical circuits to the rolls 15, 17 and 18. As long a the sheet is between both rolls 14 and 15 and rolls 17 and 18, both switch 30 and switch 33 are in closed position.

As the trailing end of the sheet passes the tripping arm 32, the switch 30 is opened by the spring tension on its tripping arm. However, by the time this occurs, the switch 33 has been closed by the leading edge of the sheet, so the rolls 15, 17 and 18 continue to be connected to the source of D.C. voltage 38.

As the sheet is propelled forwardly through the nip between the rolls 1! and 18, the excess toner liquid is squeezed cit of the surface of the sheet since the surface of the roll 17 is flexible. At the same time, a bias voltage is applied to the sheet. In the case or" the electrical circuit illustrated by FIGURE 3, this is a backward bias, since the roll 18 in contact with the surface or" the sheet carrying the electrostatic image is negatively charged and, therefore, tends to retard the deposition of positivelycharged pigment particles on the surface of the sheet. As explained hereinbefore, this action tends to produce a clean background by minimizing the pick-up of the positively-charged pigment particles by any residual negative electrostatic carried by those areas.

As e sheet leaves the nip between the rolls 1'7 and 18, the sl. t is guided by the sweep 19 onto the conveyor belts 2% 24) which deliver the toned sheet outside the apparatus. The completed copy is still damp with residual, volatile solvent of the toner liquid when delivered outside the apparatus, when it has been processed at a relatively-rapid rate, but is not wot with the solvent due to the squeezing action of the rolls 17 and 18 in removing the toner lipid from the sheet. The residual solvent evaporates from the eet quite rapidly upon exposure to air, due to tne volatility of the solvent. Thus, the process produces what is, in effect, a dry copy which is ready for immediate u e.

As the trailing edge of the sheet passes the tripping arm of the switch 33, the tripping arm opens the switch 33 under its own spring tension. This opening of the switch 33 disconnects the rolls 15, l? and 13 from the source of DC. voltage 38 and, therefore, turns off the voltage.

in the fore oing, it will be noted that the bias voltage is applied to the rolls i7 and 18 by the edge of a sheet contacting the trip arm 32, and is turned oil by the passing of the trailing edge of the sheet over the trip arm 32. The period of time during which the bias voltage is applied to the rolls 17 and 18 when a photoelectrostatic sheet is not between them is determined, first, by the speed at which the sheet is propelled through the toning apparatus and, second, the total of the distance between the tripping arm 32 of the switch 30 and the nip between the rolls 17 and 18 and that between the nip between those rolls and the tripping arm 35 or" the switch 33.

A typical apparatus of the type in which office copies are produced will move the photoelectrostatic sheets through the apparatus at a speed of about ten feet per minute. in a toning unit in accordance with this invention which the arcuate path of travel of the sheet between tripping arms and 35 may, for example, ap proximate ten inches. When used in an apparatus in which sheets are conveyed at the rate of ten feet per minute, the bias volta e will be applied to the rolls 1? and i3 a period of about 72 seconds, when the sheet is between the rolls.

Thus, even when processing cut sheets, the proportion the time during which the bias voltage is applied to the rolls 17 and when a photoelectrostatic sheet is not between them, is relatively short. This is an advantageous feature of th s apparatus since the flow of current between the rolls when a photoelectrostatic sheet, with its high-insulating CiTfiCi, is not between them is disadvantageous in that it causes a transfer of pigment between the rolls which causes a soiling of the following copy sheets. In general, it is desirable to restrict the period during which the bias voltage is on the rolls 17 and 18 without a photoelectrostatic sheet between them, to not more than two minutes, when the toning unit forms a part of apparatus intended to process cut sheets of photoelectrostatic paper.

in the case of apparatus which is designed to produce copies on continuous webs of photoelectrostatic sheet material, this time factor becomes immaterial since the bias voltage can be turned on the rolls 17 and 18 as the leading edge of the roll of material enters the toning unit, left on continuously until the trailing edge leaves the unit.

The roll 15 which first contacts the reverse surface of a sheet being toned, carries the full positive potential of the bias voltage source, in two of the four alternativetoning, preferred procedures which may be carried out by this method and apparatus. This positive potential neutralizes the negative electrostatic charge carried by the back of the sheet much more efiectively than a roll at ground potential, such as has been heretofore suggees-ed. This is an advantageous feature which becomes highly important to uniform toning in a compact apparatus in which the trailing end of a sheet is still receiving a negative electrostatic charge from a corona discharge e the leading end of the sheet is within the toning unit.

Although the application of a positive voltage to the roll 1:; in the toning of a negative electrostatic image is advantageous, it is not essential to securing the advantageous features of the floating bias voltage to the rolls 1'7 and 1-3, as described hereinbefore. A definite advantage arises from this feautre of the method and apparatus, in that it is effective in the toning of a photoelectrostatic sheet which has a resistivity of not more than 10 ohms at the nip between the rolls 14 and 15. The prior method for the application of bias voltage, in which the rolls contacting the reverse surface of the sheet are at ground potential, requires that the sheet have a resistivity no higher than 10 ohms.

The embodiment of this invention in which roll 17 has a resilient surface, provides another advantage arising from the fact that the rolls 1'7 and 18 act as squeeze rolls to eliminate excess li-v id toner from the surface of the sheet, eliminating the necessity for a second set or rolls for this purpose. Thus, the rolls 17 and 18 serve the dual purpose or providing a bias voltage to improve the toning of the image and, at the same time, function as squeeze rolls, eliminating the necessity for a separate pair of squeeze rolls. This dual use of these rolls resulted from the observation that the use of the rolls as squeeze rolls, contrary to a reasonable expectation based on the theorized mechanism by which a bias voltage improves the toning of an image, interferes with that mechanism.

As already noted, the detailed description of the apparatus illustrated by FlGUr ES 1, 2 and 3, and the method carried out by its use is directed to the toning of a negative electrostatic image while applying a backward bias voltage. The apparatus may be readily moditied to apply a forward bias voltage in such toning, by reversing the electrical connections of the rolls 1? and 18. This may be done in either of two Ways. The electrical line 42 may be connected to the roll 17 instead of to roll it and the line 3') connected to roll 18 instead of to roll 17. Alternatively, the resistor 46 may be left connected in the circuit to roll 17 by substituting a connection of the line 42 for that of line 39 to the resistor 40, and connecting line 42 to roll 18 as a substitute for its connection to line 32, illustrated by FEGURE 3.

In the foregoing, details and specific illustrations have been given for the purpose of fully describing the apparatus in accordance with this invention. However, it will be understood by those skilled in the art, that many variations can be made in the details which have been given without departing from the spirit of the invention or the scope of the claims which follow.

I claim:

1. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface;

a means for Wetting the image-bearing surface of a photoelectrostatic sheet with a toning liquid;

a means for guiding an edge of the photoelectrostatic sheet, after its image-bearing surface has been wet with toning liquid, into the nip between the said rolls with the image-bearing face of the sheet facing the said roll having the smooth, non-yielding surface;

a source of DC. voltage;

an electrical line connecting one of the poles of the said source of DC. voltage and the said roll which has the smooth, non-yielding surface;

an electrical line connecting the other pole of the source of DC. voltage and the said roll which has a resilient, electrically-conductive surface;

and, a resistor in at least one of the lines connecting the said source of DC. voltage with the said rolls.

2. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electricallyconductive surface;

a means for wetting the image-bearing surface of a photoelectrostatic sheet with a toning liquid;

a means for guiding an edge of the photoelectrostatic sheet after its image-bearing surface has been wet with toning liquid, into the nip between the said rolls, with the image-bearing face of the sheet facing the said roll having the smooth, non-yielding surface;

a source of DC. voltage;

an electrical line connecting the negative pole of the said source of DC. voltage and the said roll which has the smooth, non-yielding surface;

an electrical line connecting the positive pole of the source of DC. voltage and the said roll which has a resilient, electrically-conductive surface,

and a resistor in the line connecting the positive pole of the source of DC. voltage with the said roll which has a resilient surface.

3. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface;

a means for wetting the image-bearing surface of a photoelectrostatic sheet with a toning liquid;

a means for guiding an edge of the photoelectrostatic sheet, after its image-bearing surface has been wet with toning liquid, into the nip between the said rolls with the image-bearing face of the sheet facing the said roll having the smooth, non-yielding surface;

a source of DC. voltage which is adjustable to provide a voltage within the range of about vol'tsto about 300 volts;

an electrical line connecting the negative pole of the said source of DC. voltage and the said roll which has the smooth, non-yielding surface;

an electrical line connecting the positive pole of the source of DC. voltage and the said roll which has a resilient, electrically-conductive surface;

and a resistor having an electrical resistance within the range of about 1,000 ohms to about one megohm in the lines connecting the said source of D.V. voltage with the said roll which has a resilient surface.

4. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the said rolls with its electrostatic, image-bearing surface facing the electrically non-conductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface;

a means for wetting the image-bearing surface of the photoelectrostatic sheet with a toning liquid;

a means for guiding the leading edge of a sheet from the nip of the first pair of rolls into the nip between the second pair of rolls with its electrostatic, imagebearing surface facing the roll having the non-yielding surface;

a source of DC. voltage;

an electrical line connecting one pole of the said source of DC. voltage and both the electrically-conductive roll of the first pair of rolls and the roll of the sec ond pair of rolls which has the electrically-conductive, resilient surface;

a second electrical line connecting the other pole of the said source of DC. voltage with the roll of the second pair of rolls which has a non-yielding surface;

a resistor in at least one of the lines connecting the said source of DC. voltage with the said rolls of the second pair,

and means for connecting and disconnecting said source of DC voltage to the said rolls.

5. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the said rolls with its electrostatic, image-bearing surface facing the electrically non-conductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface; 7

a means for wetting the image-bearing surface of the photoelectrostatic sheet with a toning liquid;

a means for guiding the leading edge of a sheet from the nip of the first pair of rolls into the nip between the second pair of rolls with it's electrostatic, imagebearing surface facing the roll having the non-yielding surface;

a source of DC. voltage;

an electrical line connecting the positive pole of the said source of DC. voltage, and both the electrically-conductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electricallyconductive, resilient surface;

a second electrical line connecting the negative pole of the said source of DC. voltage with the roll of the isecond pair of rolls which has a non-yielding surace; 1

a resistor in the line connecting the positive pole of the said source of DC. voltage with the said roll which has a resilient surface,

and means for connecting and disconnecting the said source of DC. voltage to the said rolls.

6. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the said rolls with its electrostatic, image-bearing surface facing the electrically non-conductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, nonyielding surface and the other of which has a resilient, electrically-conductive surface which has a resistance of not more than 10 ohms;

a means for wetting the image-bearing surface of the photoelectrostatic sheet with a toning liquid;

an electrical line connecting the positive pole of the said source of DC. voltage and both the electrically-conductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electricallyconductive, resilient surface;

second electrical line connecting the negative side of the said source of DC. voltage with the roll of the second pair of rolls which has a non-yielding surface;

resistor having an electrical resistance within the range of about 1,000 ohms to about one megohm in the line connecting the positive pole of the said source of DC. voltage with the said roll which has a resilient surface,

and means for automatically connecting and disconimage on a photoeiectrostatic sheet, which comprises:

first pair of rolls, one of which is electrically nonconductive and the other of which is electricaily conductive;

transport means for guiding a photoeiectrostatic sheet into the nip between the said rolls with its electrostatic, irnage-bearing surface facing the electrically non-conductive roll;

second pair of rolls, both of which are electrically conductive and one of which has a smooth, nonyielding surface, the other of which has a resilient, electrically-conductive surface which has an electrical resistance of not more 26 ohms;

means for wetting the image-bearing sur cc of the photoclectrostatic sheet with a toning liquid; means for guiding the leading edge of a sheet from the nip of the first pair of rolls into the r between the second pair of rolls with its electrostatic, imagebearing surface facing the roll having the non-yielding surface;

vide a voltage within the range of about it) volts to about 309 volts;

an electrical line connecting the positive of he said source of llC. voltage and both the electricallyconductive roll of the first pair of rolls and the roll of the second pair of rolls which the electricallyconductive, resilient surface;

second electrical tine connectin the negative side of the said source of DC. voltage with the rolls of the second pair of rolls which has a non-yielding surface;

resistor having an electrical resistance within the range of about 1,060 ohms to about one meg-ohm in the line connecting the positive do of the s id source of llC. voltage with the said roll which has a resilient surface,

and means for connecting and disconnecting the said 8. image on a photoelectrostatic sheet, which co source of 11C. voltage to the said rolls which are first pair of rolls, one of which is elec' 'ca conductive and the other of which is electrically conductive;

l litransport means for guiding a photoelectrostatic sheet into the nip between the rolls with its electrostatic, image-hearing surface facing the electrically nonconductive roll;

second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface and is positioned below the other said roll of the pair;

an arcuate sheet guide having its concave surface facing upwardly, having one end located adjacent the first pair of rolls in a position to receive the leading edge of a photoelectrostatic sheet and to guide the sheet along an arcuate path with its image-bearing surface facing upwardly, and having its other end adiacent the second pair of rolls in a position to an electrical line connecting one pole of the said source and a see the 9. image on a photoelectrostatic sheet, which comprises:

of DC. voltage and both the electrically-conductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electrically-conductive, resilient surface;

second electrical line connecting the other of said source of DC. voltage with the other roll of the second pair of rolls which has a non-yielding surface; resistor at least one of the lines connecting the said source of 11C. voltage with the said rolls of the second pair;

first switch located in one of the said electrical lines connected to the source of DC. voltage adjacent the exit side of the first pair of rolls, having a tripping arm located adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

Ed switching electrically connected in parallel with the first switch adjacent the exit side of the second pair of rolls adiacent the nip between the rolls, in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes -'ch, keeps the switch closed during the period in whun the sheet is passin in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

said sw tches connecting the source of D.C. voltage 0 the said rolls when either or both are in closed position, and disconnecting the said source of DC. voltage when both are in open position.

Apparatus for the development of an electrostatic a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the rolls with its electrostatic, image-hearing surface facing the electrically non-conductive roll;

second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface and is positioned below the other said roll or" the pair;

an arcuate sheet guide having its concave surface facing upwardly, having one end located adjacent the first pair of rolls in a position to receive the leading edge of a photoelectrostatic sheet and to guide the sheet along an arcuate path with its image-bearing surface facing upwardly, and having its other end adjacent the second pair of rolls in a position to guide the leading edge of a sheet traversing its surface into the nip betwen the said second pair of rolls with its image-bearing surface facing the non-resilient roll of the said pair of rolls;

a conduit for toning liquid, having perforations along the length of its lower periphery, located above and extending traversely of the said arcuate sheet guide, which is adapted to spnay the surface of a photoelectrostatic sheet passing over the sheet guide;

a source of D.C. voltage;

an electrical line connecting the positive pole of the said source of D.C. potential and both the electricallyconductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electricallyconductive, resilient surface; a second electrical line connecting the negative pole of the said source of D.C. voltage with the other roll of the second pair of rolls which has a non-yielding surface;

a resistor in at least one of the lines connecting the said source of DC. voltage with the said rolls of the second pair;

a first switch located in one of the said lines connected to the source of DC. voltage adjacent the exit side of the first pair of rolls having a tripping arm located adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photo electrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

and a second switch electrically connected in parallel with the first switch adjacent the exit side of the second pair of rolls adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

the said switches connecting the source of D.C. voltage to the said rolls when either or both are in closed position and disconnecting the said source of D.C. voltage when both are in open position.

10. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the rolls with its electrostatic, image-bearing surface facing the electrically non-conductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface and is positioned below the other'said roll of the pair;

an arcuate sheet guide having its concave surf-ace facing upwardly, having one end located adjacent the first pair of rolls in a position to receive the leading edge of a photoelectrostatic sheet and to guide the sheet along an arcuate path with its image-bearing surface facing upwardly, and having its other end adjacent the second pair of rolls in a position to guide the leading edge of a sheet traversing its surface into the nip betwen the said second pair of rolls with its image-bearing surface facing the non-resilient roll of the said pair of rolls;

a conduit for toning liquid, havinglperfor'ations along the length of its lower periphery, located above and extending traversely of the said arcuatesheet guide, which is adapted to spray the surface of a photoelectrostatic sheet passing over the sheet guide;

a source of D.C. voltage;

an electrical line connecting the-positive pole ofthe said source of D.C. voltage and both the electricallyconductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electricallyconductive, resilient surface; a second electrical line connecting the negative pole of the said source of D.C. voltage with the other roll of the second pair of rolls which has a non-yielding surface;

a resistor in at least one of the lines connecting the saidsource of D.C. voltage with the said rolls of the second pair; 7

a first switch located in one of the said lines connected to the source of D.C. voltage adjacent the exit side of the first pair of rolls having a tripping arm located adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

and a second switch electrically connected in parallel with the first switch adjacent the exit side of the second pair of rolls adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

the said switches connecting the source of D.C. voltage to the said rolls when either or both are in closed position and disconnecting the said source of D.C. voltage when both are in openposition.

11. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding a photoelectrostatic sheet into the nip between the rolls with its electrostatic, image-bearing surface facing the electrically nonconductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface and is positioned below the other said roll of the pair;

an arcuate sheet guide having its concave surface facing upwardly, having one end located adjacent the first pair of rolls in a position to receive the leading edge of a photoelectrostatic sheet and to guide the sheet along an arcuate path with its image-bearing surface facing upwardly, and having its other end adjacent the second pair of rolls in a position to guide the leading edge of a sheet traversing its surface into the nip between the said second pair of rolls with its image-bearing surface facing the non resilient roll of the said pair of rolls;

a conduit for toning liquid, having perforations along the length of its lower periphery, located above and extending traversely of the said arcuate sheet guide, which is adapted to spray the surface of a photoelectrostatic sheet passing over the sheet guide;

a source of D.C. voltage which is adjustable to provide a voltage Within the range of about 10 volts to about 300 volts;

an electrical line connecting the positive pole of the said source of D.C. voltage and both the electricallyconductive roll of the first pair of rolls and the roll 1 7 of the second pair of rolls which has the electricallyconductive, resilient surface; a second electrical line connecting the negative pole of the said source of DC. voltage with the other roll of the second pair of rolls which has a non-yielding surface;

a resistor having an electrical resistance within the range of about 1,000 ohms to about one megohrn in the line connecting the said source of DC. voltage to the roll of the second pair which has a resilient surface;

a first switch located in one of the said lines connected to the source of DC voltage adjacent the exit side of the first pair of rolls having a tripping arm located adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

and a second switch electrically connected in parallel with the first switch adjacent the exit side of the second pair of rolls adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

the said switches connecting the source of D.C. voltage to the said rolls when either or both are in closed position and disconnecting the said source of DC. voltage when both are in open position.

12. Apparatus for the development of an electrostatic image on a photoelectrostatic sheet, which comprises:

a first pair of rolls, one of which is electrically nonconductive and the other of which is electrically conductive;

a transport means for guiding photoelectrostatic sheet into the nip between the rolls with its electrostatic image-bearing surface facing the electrically nonconductive roll;

a second pair of rolls, both of which are electrically conductive, one of which has a smooth, non-yielding surface and the other of which has a resilient, electrically-conductive surface which has an electrical resistance of not more than 10 ohms and is positioned below the other said roll of the pair;

an arcuate sheet guide having its concave surface facing upwardly, having one end located adjacent the first pair of rolls in a position to receive the leading edge of a photoelectrostatic sheet and to guide the sheet along an arcuate path with its image-bearing surface facing upwardly, and having its other end adjacent the second pair of rolls in a position to guide the leading edge of a sheet traversing its surface into the nip between the said second pair of rolls with its image-bearing surface facing the non-resilient roll of the said pair of rolls;

a conduit for toning liquid, having perforations along the length of its lower periphery, located above and extending traversely of the said arcuate sheet guide, which is adapted to spray the surface of a photoelectrostatic sheet passing over the sheet guide;

a source of DC. voltage which is adjustable to provide a voltage within the range of about 10 volts to about 300 volts;

an electrical line connecting the positive pole of the said source of DC. voltage and both the electricallyconductive roll of the first pair of rolls and the roll of the second pair of rolls which has the electricallyconductive, resilient surface; a second electrical line connecting the negative pole to the said source of DC. voltage with the other roll of the second pair of rolls which has a non-yielding surface;

a resistor having an electrical resistance within the range of about 1,000 ohms to about one megohm in the line connecting the said source of DC. voltage to the roll of the second pair of rolls which has a resilient surface;

a first switch located in the line connecting the said source of DC. voltage to the roll of the second pair of rolls which has a non-yielding surface, adjacent the exit side of the first pair of rolls having a tripping arm located adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

and a second switch electrically connected in parallel with the first switch adjacent the exit side of the second pair of rolls adjacent the nip between the rolls in a position such that it is contacted by the leading edge of a photoelectrostatic sheet and closes the switch, keeps the switch closed during the period in which the sheet is passing in contact with it, and then permits the switch to open as the trailing edge of the sheet passes it;

the said switches connecting the source of DC. voltage to the said rolls when either or both are in closed position and disconnecting the said source of DC. voltage when both are in open position.

References Cited in the file of this patent UNITED STATES PATENTS 2,884,704 Bolton May 5, 1959 2,979,026 Reuter Apr. 11, 1961 2,991,754 Johnson July 11, 1961 3,038,073 Johnson June 5, 1962 

1. APPARATUS FOR THE DEVELOPMENT OF AN ELECTROSTATIC IMAGE ON A PHOTOELECTROSTATIC SHEET, WHICH COMPRISES: A PAIR OF ROLLS, BOTH OF WHICH ARE ELECTRICALLY CONDUCTIVE, ONE OF WHICH HAS A SMOOTH, NON-YIELDING SURFACE AND THE OTHER OF WHICH HAS A RESILIENT, ELECTRICALLY-CONDUCTIVE SURFACE; A MEANS FOR WETTING THE IMAGE-BEARING SURFACE OF A PHOTOELECTROSTATIC SHEET WITH A TONING LIQUID; A MEANS FOR GUIDING AN EDGE OF THE PHOTOELECTROSTATIC SHEET, AFTER ITS IMAGE-BEARING SURFACE HAS BEEN WET WITH TONING LIQUID, INTO THE NIP BETWEEN THE SAID ROLLS WITH THE IMAGE-BEARING FACE OF THE SHEET FACING THE SAID ROLL HAVING THE SMOOTH, NON-YIELDING SURFACE; A SOURCE OF D.C. VOLTAGE; AN ELECTRICAL LINE CONNECTING ONE OF THE POLES OF THE SAID SOURCE OF D.C. VOLTAGE AND THE SAID ROLL WHICH HAS THE SMOOTH, NON-YIELDING SURFACE; AN ELECTRICAL LINE CONNECTING THE OTHER POLE OF THE SOURCE OF D.C. VOLTAGE AND THE SAID ROLL WHICH HAS A RESILIENT, ELECTRICALLY-CONDUCTIVE SURFACE; AND, A RESISTOR IN AT LEAST ONE OF THE LINES CONNECTING THE SAID SOURCE OF D.C. VOLTAGE WITH THE SAID ROLLS. 